1. Field of the Invention
The present invention relates to processes for preparing hydrofluoroolefins such as tetrafluorinated propenes.
2. Description of the Related Art
Fluorocarbon based fluids have found widespread use in many commercial and industrial applications including as refrigerants, aerosol propellants, blowing agents, heat transfer media, gaseous dielectrics and as working fluids in air conditioning, heat pump and refrigeration systems. The vapor compression cycle is one of the most commonly used type methods to accomplish cooling or heating in a refrigeration system. The vapor compression cycle usually involves the phase change of the refrigerant from the liquid to the vapor phase through heat absorption at a relatively low pressure and then from the vapor to the liquid phase through heat removal at a relatively low pressure and temperature, compressing the vapor to a relatively elevated pressure, condensing the vapor to the liquid phase through heat removal at this relatively elevated pressure and temperature, and then reducing the pressure to start the cycle over again. Certain fluorocarbons have been a preferred component in many heat exchange fluids. For, example, fluoroalkanes, such as chlorofluoromethane and chlorofluoroethane derivatives, have gained widespread use as refrigerants in applications including air conditioning and heat pump applications owing to their unique combination of chemical and physical properties. Many of the refrigerants commonly utilized in vapor compression systems are either single components fluids or azeotropic mixtures.
However, suspected environmental problems associated with the use of some of these fluids, including the relatively high global warming potentials associated therewith, it is desirable to use fluids having the lowest possible greenhouse warming potential in addition to zero ozone depletion potential. Thus there is considerable interest in developing environmentally friendlier materials for the applications mentioned above. Concern has increased in recent years about potential damage to the earth's atmosphere and climate, and certain chlorine-based compounds have been identified as particularly problematic in this regard. The use of chlorine-containing compositions (such as chlorofluorocarbons (CFC's), hydrochlorofluorocarbons (HCF's) and the like) as refrigerants in air-conditioning and refrigeration systems has become disfavored because of the ozone-depleting properties associated with many of such compounds. Thus, there is an increased need for new fluorocarbon and hydrofluorocarbon compounds and compositions that offer alternatives for refrigeration and heat pump applications. For example, it has become desirable to retrofit chlorine-containing refrigeration systems by replacing chlorine-containing refrigerants with non-chlorine-containing refrigerant compounds that will not deplete the ozone layer, such as hydrofluorocarbons (HFC's).
Although most HFC's will not deplete the ozone layer, there is concern about the global warming potential (GWP) associated with these molecules. GWP is a measure of the potential contribution to the green house effect of the chemical against a reference, the reference molecule in this case is CO2 which has a GWP=1. Regulation in the European Union has already set limits of a GWP=150 for refrigerants to be used in some applications such as automotive air conditioning.
Hydrofluorocarbons (HFC's), particularly hydrofluoroalkenes such tetrafluoropropenes, have been disclosed to be effective refrigerants, fire extinguishants, heat transfer media, propellants, foaming agents, blowing agents, gaseous dielectrics, sterilant carriers, polymerization media, particulate removal fluids, carrier fluids, buffing abrasive agents, displacement drying agents and power cycle working fluids. Unlike chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), both of which potentially damage the Earth's ozone layer, HFCs do not contain chlorine and thus pose no threat to the ozone layer. Tetrafluoropropenes, having zero ozone depletion and low global warming potential, have been identified as potentially filling this need. However, the toxicity, boiling point, and other physical properties in this class of chemicals vary greatly from isomer to isomer. One tetrafluoropropene having valuable properties is 2,3,3,3-tetrafluoropropene (HFO-1234yf). Thus, there is a need for new manufacturing processes for the production of tetrafluoropropenes and in particular 2,3,3,3-tetrafluoropropene. 2,3,3,3-Tetrafluoropropene (HFO-1234yf) is a refrigerant that has been designed to meet the EU regulations for automotive air conditioning.
U.S. patent application publication US2007/0179324 discloses a synthesis process involving the dehydrofluorination of 1,1,1,2,3-pentafluoropropane (HFC-245eb) to form HFO-1234yf. Also disclosed is the dehydrofluorination of 1,1,1,2,3,3-hexafluoropropane (HFC-236ea) under similar conditions to form 1,2,3,3,3-pentafluoropropene (HFO-1225ye(Z)). Accordingly, the presence of HFC-236ea impurities in this reaction could produce HFO-1225ye(Z) as an unwanted byproduct. To overcome this issue, US2007/0179324 also discloses that HFO-1225ye(Z) can be reacted with hydrogen to form HFC-245eb, which can be dehydrofluorinated to form HFO-1234yf. However, any HFO-1225ye(Z) which is not fully hydrogenated in the first step will end up as an impurity if the final product stream of HFO-1234yf.
Distillation is a conventional purification method for commercial processes. Distillation takes advantage of the fact that the liquid and vapor of a chemical mixture are at different compositions. However, distillation becomes inefficient and then impossible as the chemical mixture approaches a pinch point and an azeotropic composition, respectively. The closer the composition is to a pinch point the more yield loss there will be in the distillation. This phenomena renders the separation of HFO-1225ye(Z) from a HFO-1234yf product stream using standard distillation techniques very difficult. More particularly, it is very difficult to remove HFO-1225ye(Z) from HFO-1234yf by conventional distillation at very low levels of HFO-1225ye(Z) without large yield losses due to the fact that HFO-1225ye(Z) and HFO-1234yf form an azeotrope (see U.S. Pat. No. 7,098,176).